Atherosclerosis is a chronic inflammatory disease that results from hyperlipidemia and a complex interplay of a variety of environmental, metabolic and genetic risk factors. The oxidation of low density lipoprotein (OxLDL) plays a central, if not obligatory role, in the atherogenic process. Early studies showed that acetylation of LDL greatly enhanced its uptake by macrophages and that the uptake occurred via “scavenger receptors” which were distinct from the classical LDL receptor. Unlike most receptors, these scavenger receptors were not downregulated following uptake of OxLDL. Due to the excessive uptake of OxLDL and its associated lipid by the macrophages, the cells obtained a characteristic foam-like appearance. The appearance of such cells is one of the first hallmarks of atherosclerotic disease. Foam cells accumulate within the intima (under the endothelial lining) of the vessel walls where they lead to plaque formation, the hallmark of more advanced disease. Inflammatory conditions develop leading to the development of complicated lesions.
There is much evidence that OxLDL contributes to atherogenesis by a number of mechanisms. The oxidation of polyunsaturated fatty acids in phospholipids of lipoproteins generates many breakdown products such as malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), and other reactive moieties attached to oxidized phospholipids. Many of these intermediate products are highly reactive and can interact with lysine residues of associated proteins and phospholipids to generate various adducts. These adducts are known to occur in vivo and are immunogenic. In murine models of atherosclerosis, such as apo-E deficient mice (ApoE−/−) mice, atherosclerosis is correlated with the development of high titers of autoantibodies to various oxidation specific epitopes of OxLDL. The consequences of such cellular and humoral responses are still poorly understood, but under certain conditions they can clearly modify the natural history of the disease.
It is generally accepted that it is the composition of atherosclerotic lesions, in particular the content of lipids, OxLDL, foam cells, and smooth muscle cells that determines their properties. Foam cells are often found in the sites of lesion that are susceptible to rupture. Activated macrophages recruited to clear the apoptotic and necrotic foam cells, as well as OxLDL, secrete factors that weaken the plaque. Human pathology studies have shown that atheromas containing a large necrotic core, thin fibrous cap and large numbers of macrophage/foam cells in the shoulder are more predisposed to plaque rupture and thrombosis. These lesions, which frequently appear as mild or moderate coronary stenoses in angiographic studies, are characterized pathologically as large atheroma with extensive lipid pools exceeding 40% of plaque areas. Angiography only provides a measure of arterial lumen, but fails to detect vessel wall pathology. Diagnostic methods that provide a measure of the overall extent of the atherosclerotic lesion, with an emphasis on OxLDL and lipid content, would therefore be desirable. Moreover, the lipid core of atheromas can be assumed to contain extensive oxidized lipids that accumulated within foam cells and set free when cells undergo necrosis and apoptosis.
Non-invasive detection of atherosclerotic lesions can now be performed in animal models using imaging techniques that rely on antibodies that are specific for OxLDL. (See, for example, U.S. Pat. No. 6,716,410). Human studies have suggested that plaque rupture frequently occurs in non-angiographically significant lesions that contain abundant lipid-laden macrophages and large lipid pools within atheromas. Therefore imaging of atherosclerosis directed at lipid rich areas is of value, not only in detecting the extent of lesion burden, but also in the detecting clinically silent but “active” lesions.
In addition to such imaging techniques, there exists a need to develop simple noninvasive ways of studying atherogenesis that relate to the complexities of plaque biology rather than on plaque architecture or lipid profiles as a whole. Accordingly, the present invention relates to a plasma biomarker that specifically reflects plaque biology associated with atherogenesis.